Pressure-sensitive adhesive with enhanced resistance to water-whitening

ABSTRACT

A pressure-sensitive adhesive comprising an aqueous polymer dispersion prepared by emulsion polymerization, wherein the polymer comprises monomers containing a hydroxyl group or primary amino group (referred to below collectively for short as hydrophilic monomers) and the polymer is obtainable by supplying more than 70% by weight of the hydrophilic monomers only when the polymerization mixture (mixture present in the polymerization vessel and composed of monomers and polymer already formed) already comprises more than 50% by weight of the total amount of monomers.

The invention relates to a pressure-sensitive adhesive comprising anaqueous polymer dispersion prepared by emulsion polymerization, whereinthe polymer comprises monomers containing a hydroxyl group or primaryamino group (referred to below collectively for short as hydrophilicmonomers) and the polymer is obtainable by supplying more than 70% byweight of the hydrophilic monomers only when the polymerization mixture(mixture present in the polymerization vessel and composed of monomersand polymer already formed) already comprises more than 50% by weight ofthe total amount of monomers.

Self-adhesive articles are composed essentially of a backing applied towhich there is a layer of pressure-sensitive adhesive. For manyapplications the backing is transparent. Water exposure is oftenaccompanied by the phenomenon known as “blushing”, by which is meant aclouding in the adhesive layer caused by water penetration. Thisblushing detracts from the performance properties; naturally, it is inthe case of transparent film-backed labels in particular that theclouding adversely affects the visual appearance.

EP-A 1 378 527, EP-A 623 659 or WO 98/44064, for example, discloseslessening the blushing in the case of aqueous polymer dispersions bymeans of specific synthesis components of the dispersed polymer. Theresult achieved is still not entirely satisfactory.

It was an object of the present invention, therefore, to lessen or avoidthe effect of blushing while not adversely affecting but if possibleeven improving the other performance properties of thepressure-sensitive adhesive, in particular the adhesion and cohesion.

It is to this effect that the invention provides the pressure-sensitiveadhesive defined at the outset.

The pressure-sensitive adhesive comprises a polymer dispersion preparedby emulsion polymerization. The emulsion addition polymer, calledpolymer for short, is composed preferably of at least 40%, morepreferably at least 60%, and very preferably at least 80% by weight ofwhat are called principal monomers.

The principal monomers are selected from C1-C20 alkyl (meth)acrylates,vinyl esters of carboxylic acids comprising up to 20 carbon atoms,vinylaromatics having up to 20 carbon atoms, ethylenically unsaturatednitrites, vinyl halides, vinyl ethers of alcohols comprising 1 to 10carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and oneor two double bonds, or mixtures of these monomers.

Examples include (meth)acrylic acid alkyl esters having a C1-C10 alkylradical, such as methyl methacrylate, methyl acrylate, n-butyl acrylate,ethyl acrylate, and 2-ethylhexyl acrylate.

Also suitable in particular are mixtures of the (meth)acrylic acid alkylesters.

Vinyl esters of carboxylic acids having 1 to 20 carbon atoms are, forexample, vinyl laurate, vinyl stearate, vinyl propionate, Versatic acidvinyl esters, and vinyl acetate.

Suitable vinylaromatic compounds include vinyltoluene, a- andp-methylstyrene, a-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene,and, preferably, styrene. Examples of nitriles are acrylonitrile andmethacrylonitrile.

The vinyl halides are chlorine-, fluorine- or bromine-substitutedethylenically unsaturated compounds, preferably vinyl chloride andvinylidene chloride.

Examples of vinyl ethers include vinyl methyl ether or vinyl isobutylether. Preference is given to vinyl ethers of alcohols comprising 1 to 4carbon atoms.

Hydrocarbons having 2 to 8 carbon atoms and one or two olefinic doublebonds include ethylene, propylene, butadiene, isoprene, and chloroprene.

Preferred principal monomers are the C1 to C10 alkyl acrylates andmethacrylates, especially C1 to C8 alkyl acrylates and methacrylates,and vinylaromatics, especially styrene, and mixtures thereof.

Very particular preference is given to methyl acrylate, methylmethacrylate, ethyl acrylate, n-butyl acrylate, n-hexyl acrylate, octylacrylate, and 2-ethylhexyl acrylate, styrene, and mixtures of thesemonomers.

Preferably the polymer is composed of at least 40%, in particular atleast 60%, and very preferably at least 80% by weight of C1-C20,especially C1-C10 alkyl (meth)acrylates.

In accordance with the invention the polymer comprises monomerscontaining hydroxyl groups, primary amino groups or mixtures thereof(referred to collectively for short as hydrophilic monomers).

Monomers containing hydroxyl groups that may be mentioned in particularinclude C1-C10 hydroxyalkyl (meth)acrylates, preferably C2-C8hydroxyalkyl (meth)acrylates, more preferably C2-C4 hydroxyalkyl(meth)acrylates, examples being hydroxyethyl (meth)acrylate,hydroxybutyl (meth)acrylate and hydroxyalkyl (meth)acrylate. The hydroxyacrylates are preferred over the hydroxy methacrylates.

As monomers containing primary amino groups mention may be made of, forexample, methacrylamide or acrylamide.

The hydrophilic monomers are preferably selected from hydroxyalkyl(meth)acrylates, (meth)acrylamide or mixtures thereof.

The hydrophilic monomers are with particular preference monomerscomprising hydroxyl groups, with very particular preference the abovehydroxyalkyl (meth)acrylates.

The polymer comprises preferably in total 0.1% to 15% by weight of thehydrophilic monomers.

In particular the polymer comprises at least 0.2%, more preferably atleast 0.3%, by weight of the hydrophilic monomers. In particular thepolymer comprises not more than 10%, more preferably not more than 5%,and very preferably not more than 3%, by weight of the hydrophilicmonomers.

Besides the principal monomers and the hydrophilic monomers the polymermay comprise further monomers, examples being monomers having acidgroups (acid monomers; see above), e.g., carboxylic acid, sulfonic acidor phosphonic acid groups. Carboxylic acid groups are preferred.Examples that may be mentioned include acrylic acid, methacrylic acid,itaconic acid, maleic acid, and fumaric acid.

The amount of monomers of this kind containing acid groups is inparticular 0.1% to 5%, more preferably 0.2% to 3% by weight in thepolymer.

As further monomers mention may also be made of phenyloxyethyl glycolmono(meth)acrylate, glycidyl acrylate, glycidyl methacrylate, and amino(meth)acrylates such as 2-aminoethyl (meth)acrylate.

As further monomers mention may also be made of crosslinking monomers.

The glass transition temperature of the polymer is preferably −60 to 0°C., more preferably −60 to −10° C., and very preferably −60 to −20° C.

The glass transition temperature can be determined by typical methodssuch as differential thermoanalysis or differential scanning calorimetry(see, e.g., ASTM 3418/82, midpoint temperature).

Emulsion polymerization involves polymerizing ethylenically unsaturatedcompounds (monomers) in water using ionic and/or nonionic emulsifiersand/or protective colloids or stabilizers as surface-active compounds tostabilize the monomer droplets and the polymer particles formedsubsequently from the monomers.

A detailed description of suitable protective colloids is found inHouben-Weyl, Methoden der organischen Chemie, Volume XIV/1,Makromolekulare Stoffe [Macromolecular compounds], Georg-Thieme-Verlag,Stuttgart, 1961, pp. 411 to 420. Suitable emulsifiers include anionic,cationic, and nonionic emulsifiers. As surface-active substances it ispreferred to use emulsifiers, whose molecular weights, unlike those ofthe protective colloids, are typically below 2000 g/mol. Where mixturesof surface-active substances are used the individual components must, aswill be appreciated, be compatible with one another, something which incase of doubt can be checked by means of a few preliminary tests. It ispreferred to use anionic and nonionic emulsifiers as surface-activesubstances. Common accompanying emulsifiers are, for example,ethoxylated fatty alcohols (EO degree: 3 to 50, alkyl radical: C₈ toC₃₆), ethoxylated mono-, di-, and trialkylphenols (EO degree: 3 to 50,alkyl radical: C₄ to C₉), alkali metal salts of dialkyl esters ofsulfosuccinic acid and also alkali metal salts and ammonium salts ofalkyl sulfates (alkyl radical: C₈ to C₁₂), of ethoxylated alkanols (EOdegree: 4 to 30, alkyl radical: C₁₂ to C₁₈), of ethoxylated alkylphenols(EO degree: 3 to 50, alkyl radical: C₄ to C₉), of alkylsulfonic acids(alkyl radical: C₁₂ to C₁₈), and of alkylarylsulfonic acids (alkylradical: C₉ to C₁₈).

Further suitable emulsifiers are compounds of the general formula II

in which R5 and R6 are hydrogen or C4 to C14 alkyl and are notsimultaneously hydrogen, and X and Y can be alkali metal ions and/orammonium ions. Preferably R5 and R6 are linear or branched alkylradicals having 6 to 18 carbon atoms or hydrogen, and in particularhaving 6, 12 and 16 carbon atoms, R5 and R6 not both simultaneouslybeing hydrogen. X and Y are preferably sodium, potassium or ammoniumions, with sodium being particularly preferred. Particularlyadvantageous compounds 11 are those in which X and Y are sodium, R5 is abranched alkyl radical having 12 carbon atoms, and R6 is hydrogen or R5.It is common to use technical mixtures having a fraction of 50% to 90%by weight of the monoalkylated product, an example being Dowfax

2A1 (trade mark of the Dow Chemical Company).

Suitable emulsifiers are also found in Houben-Weyl, Methoden derorganischen Chemie, Volume 14/1, Makromolekulare Stoffe, Georg ThiemeVerlag, Stuttgart, 1961, pages 192 to 208.

Emulsifier trade names are, for example, Dowfax

2 A1, Emulan

NP 50, Dextrol

OC 50, Emulgator 825, Emulgator 825 S, Emulan

OG, Texapon

NSO, Nekanil

904 S, Lumiten

I-RA, Lumiten E 3065, Disponil FES 77, Lutensol AT 18, Steinapol VSL,Emulphor NPS 25.

The surface-active substance is used typically in amounts of 0.1 to 10%by weight, based on the monomers to be polymerized.

In the emulsion polymerization use is made typically of water-solubleinitiators for the free-radical polymerization of the monomers.

Water-soluble initiators for emulsion polymerization are, for example,ammonium salts and alkali metal salts of peroxydisulfuric acid, e.g.,sodium peroxodisulfate, hydrogen peroxide or organic peroxides, e.g.,tert-butyl hydroperoxide.

Also suitable are what are known as reduction-oxidation (redox)initiator systems.

The redox initiator systems are composed of at least one, usuallyinorganic reducing agent and one organic or inorganic oxidizing agent.

The oxidizing component comprises, for example, the emulsionpolymerization initiators already mentioned above.

The reducing components comprise, for example, alkali metal salts ofsulfurous acid, such as sodium sulfite, sodium hydrogen sulfite, alkalimetal salts of disulfurous acid such as sodium disulfite, bisulfiteaddition compounds of aliphatic aldehydes and ketones, such as acetonebisulfite, or reducing agents such as hydroxymethanesulfinic acid andits salts, or ascorbic acid. The redox initiator systems may be usedtogether with soluble metal compounds whose metallic component is ableto exist in a plurality of valence states.

Examples of typical redox initiator systems include ascorbicacid/iron(II) sulfate/sodium peroxydisulfate, tert-butylhydroperoxide/sodium disulfite, tert-butylhydroperoxide/Na-hydroxymethanesulfinic acid. The individual components,the reducing component for example, may also be mixtures: for example, amixture of the sodium salt of hydroxymethanesulfinic acid and sodiumdisulfite.

The stated compounds are mostly used in the form of aqueous solutions,the lower concentration being determined by the amount of water that isacceptable in the dispersion and the upper concentration by thesolubility of the respective compound in water. In general theconcentration is 0.1 to 30% by weight, preferably 0.5 to 20% by weight,more preferably 1.0 to 10% by weight, based on the solution.

The amount of the initiators is generally 0.1 to 10% by weight,preferably 0.5 to 5% by weight, based on the monomers to be polymerized.It is also possible for two or more different initiators to be used forthe emulsion polymerization.

In the course of the polymerization, polymerization regulators,regulators for short, can also be used in accordance with the invention.Regulators bring about a chain termination reaction and hence reduce themolar weight of the polymer. In the course of this reaction theregulators are attached to the polymer, generally to the chain end.

The amount of regulators can be, for example, 0 to 4 parts by weight,more preferably 0.05 to 0.8 part by weight, and very preferably 0.1 to0.6 part by weight, based on 100 parts by weight of the monomers to bepolymerized. Suitable regulators are, in particular, compounds having amercapto group, such as tert-butyl mercaptan, thioglycolic acidethylacrylic esters, mercaptoethynol, mercaptopropyltrimethoxysilane ortert-dodecyl mercaptan. The regulators are generally low molecularweight compounds having a molar weight of less than 2000, in particularless than 1000 g/mol.

A portion of the monomers can, if desired, be included in the initialcharge to the polymerization vessel at the beginning of thepolymerization; the remaining monomers, or all the monomers if nomonomers are included in the initial charge, are added in the feedprocess in the course of the polymerization.

According to the invention the polymer is obtainable by supplying morethan 70%, more preferably more than 90%, by weight of the hydrophilicmonomers only when the polymerization mixture (mixture present in thepolymerization vessel and composed of monomers and polymer alreadyformed) already comprises more than 50% by weight of the total amount ofmonomers.

With particular preference the polymer is obtainable by supplying morethan 70%, more preferably more than 90%, by weight of the hydrophilicmonomers only when the polymerization mixture (mixture present in thepolymerization vessel and composed of monomers and polymer alreadyformed) already comprises more than 70% by weight of the total amount ofmonomers.

With very particular preference the polymer is obtainable by 100% byweight of the hydrophilic monomers being added only when thepolymerization mixture (mixture present in the polymerization vessel andcomposed of monomers and polymer already formed) already comprises morethan 50%, in particular more than 70%, with very particular preferencemore than 80% by weight of the total amount of monomers.

In all references to the polymerization mixture, the term monomer shouldbe understood as including not only monomers which have not yetundergone polymerization but also monomer units of the polymer, i.e.,the copolymerized monomers.

At the beginning of the addition of the more than 70% or of the morethan 90% or 100% by weight of the hydrophilic monomers thepolymerization mixture is composed of at least 40%, in particular atleast 60%, by weight of polymer already formed.

The monomers are at least partly added continuously during thepolymerization. In part it is also possible for monomers to be includedin the initial charge to the polymerization vessel before the beginningof the polymerization.

Preferably not more than 30% by weight of the total amount of monomers,more preferably not more than 20% by weight, very preferably not morethan 10% by weight of the monomers are included in the initial charge tothe polymerization vessel.

The remaining monomers, i.e., preferably at least 70% by weight, morepreferably at least 80% by weight, with very particular preference atleast 90% by weight, are added continuously during the polymerization.In one particular embodiment no monomers are included in the initialcharge, i.e., the entirety of the monomers is run in during thepolymerization.

The temperature of the polymerization mixture during polymerization and,correspondingly, during the addition of the monomers is preferably atleast 50° C., more preferably at least 70° C.

The addition of the monomers to the polymerization vessel takes placepreferably over a period of at least two hours, more preferably at least2.5 hours.

Otherwise, the conduct of the emulsion polymerization is subject to thefollowing remarks:

The emulsion polymerization takes place in general at 30 to 130,preferably 50 to 90° C. The polymerization medium may be composed eitherof water alone or of mixtures of water and water-miscible liquids suchas methanol. Preferably only water is used. The feed process may beconducted as a staged or gradient procedure. Preference is given to thefeed process, in which a portion of the polymerization mixture isintroduced as an initial charge and heated to the polymerizationtemperature, the polymerization of this initial charge is commenced, andthen the remainder of the polymerization mixture is supplied to thepolymerization zone, typically by way of two or more spatially separatefeed streams, of which one or more comprise the monomers in straight oremulsified form, this addition being made continuously, in stages orunder a concentration gradient, and polymerization being maintainedduring said addition. It is also possible, in order, for example, to setthe particle size more effectively, to include a polymer seed in theinitial polymerization charge.

The manner in which the initiator is added to the polymerization vesselin the course of the free-radical aqueous emulsion polymerization isknown to the average skilled worker. It may either be included in itsentirety in the initial charge to the polymerization vessel or elseintroduced, continuously or in stages, at the rate at which it isconsumed in the course of the free-radical aqueous emulsionpolymerization. In each specific case this will depend both on thechemical nature of the initiator system and on the polymerizationtemperature. It is preferred to include one portion in the initialcharge and to supply the remainder to the polymerization zone at therate at which it is consumed.

In order to remove residual monomers it is common to add initiator afterthe end of the actual emulsion polymerization as well, i.e., after amonomer conversion of at least 95%.

With the feed process, the individual components can be added to thereactor from the top, through the side, or from below, through thereactor floor.

In the case of the emulsion polymerization, aqueous polymer dispersionswith solids contents of generally 15% to 75% by weight, preferably of40% to 75% by weight, are obtained.

For a high reactor space/time yield, dispersions with as high aspossible a solids content are preferred. In order to be able to achievesolids contents >60% by weight, a bimodal or polymodal particle sizeought to be set, since otherwise the viscosity becomes too high and thedispersion can no longer be handled. Producing a new generation ofparticles can be done, for example, by adding seed (EP 81083), by addingexcess quantities of emulsifier, or by adding miniemulsions. Anotheradvantage associated with the low viscosity at high solids content isthe improved coating behavior at high solids contents. One or more newgenerations of particles can be produced at any point in time. Thispoint in time depends on the particle size distribution which istargeted for a low viscosity.

The polymer thus prepared is used preferably in the form of its aqueousdispersion.

The polymer is preferably used as or in pressure-sensitive adhesives(PSAs).

The PSA comprises the polymer preferably in the form of the aqueouspolymer dispersion as has been obtained, or is obtainable, by emulsionpolymerization.

The PSA may be composed exclusively of the polymer, or of the aqueousdispersion of the polymer.

Alternatively the PSA may comprise further additives.

Suitable examples include a tackifier, i.e., a tackifying resin.Tackifiers are known for example from Adhesive Age, July 1987, pages19-23 or Polym. Mater. Sci. Eng. 61 (1989), pages 588-592.

Tackifiers are, for example, natural resins, such as rosins and theirderivatives formed by disproportionation or isomerization,polymerization, dimerization and/or hydrogenation. They may be presentin their salt form (with, for example, monovalent or polyvalentcounterions (cations)) or, preferably, in their esterified form.Alcohols used for the esterification may be monohydric or polyhydric.Examples are methanol, ethanediol, diethylene glycol, triethyleneglycol, 1,2,3-propanethiol, and pentaerythritol.

Also used are hydrocarbon resins, e.g., coumarone-indene resins,polyterpene resins, hydrocarbon resins based on unsaturated CHcompounds, such as butadiene, pentene, methylbutene, isoprene,piperylene, divinylmethane, pentadiene, cyclopentene, cyclopentadiene,cyclohexadiene, styrene, a-methylstyrene, and vinyltoluene.

Other compounds increasingly being used as tackifiers includepolyacrylates which have a low molar weight. These polyacrylatespreferably have a weight-average molecular weight M_(w) of below 30,000.The polyacrylates with preference are composed of at least 60%, inparticular at least 80% by weight of C₁-C₈ alkyl (meth)acrylates.

Preferred tackifiers are natural or chemically modified rosins. Rosinsare composed predominantly of abietic acid or abietic acid derivatives.

The tackifiers can be added in a simple way to the polymers of theinvention, preferably to the aqueous dispersions of the polymers. Inthis case the tackifiers are preferably themselves in the form of anaqueous dispersion.

The amount by weight of tackifiers is preferably 5 to 100 parts byweight, more preferably 10 to 50 parts by weight, based on 100 parts byweight of polymer (solids/solids).

Besides tackifiers, for example, further additives may find use,examples being thickeners, preferably associative thickeners, defoamers,plasticizers, pigments, wetting agents or fillers, in the case of thePSA utility.

Accordingly the PSAs of the invention further comprise, in addition tothe aqueous polymer dispersion, if appropriate, tackifiers and/or theabove additives.

For improved surface wetting the PSAs may comprise, in particular,wetting assistants, examples being fatty alcohol ethoxylates,alkylphenol ethoxylates, sulfosuccinic esters, nonylphenol ethoxylates,polyoxyethylenes/-propylenes or sodium dodecylsulfonates. The amount isgenerally 0.05 to 5 parts by weight, in particular 0.1 to 3 parts byweight, per 100 parts by weight of polymer (solids).

The PSAs are suitable for producing self-adhesive articles such aslabels, sheets or adhesive tapes. The PSA can be applied by typicalmethods, such as for example by rolling, knifecoating, spreading, etc.,to backings, examples being paper or polymeric films, composedpreferably of polyethylene, polypropylene, which may have been biaxiallyor monoaxially oriented, polyethylene terephthalate, polyvinyl chloride,polystyrene, polyamide or metal. Also suitable in particular arebackings having nonpolar surfaces, made for example of polyolefins,especially polyethylene or polypropylene, since the dispersions of theinvention adhere well to such backings.

The water can be removed preferably by drying at 50 to 150° C. Before orafter the adhesive is applied the backings may be slit to form adhesivetapes, labels or sheets. For subsequent use the PSA-coated side of thesubstrates may be lined with a release paper, e.g., with a siliconizedpaper.

The self-adhesive articles of the invention have very good adhesiveproperties, in particular an effective adhesion to the substrates and ahigh level of cohesion (internal strength in the adhesive layer).

In particular the PSAs of the invention are suitable also fortransparent backings, in particular not least for film-backed labels,since the “blushing” that frequently occurs on water exposure does notoccur, or is at least lessened. Consequently the performance propertiesare effectively retained even on water exposure; there is no—orvirtually no—clouding or other impairment of the visual appearanceobserved.

COMPARATIVE EXAMPLE 1 Without Hydrophilic Monomers

In a 4-liter polymerization reactor with anchor stirrer andheating/cooling apparatus a mixture of 223.9 g of deionized water and18.2 g of a 33% strength by weight aqueous polymer latex (prepared byfree-radically initiated emulsion polymerization of styrene) having aweight-average particle diameter Dw50 of 30 nm was heated at 85° C.under a nitrogen atmosphere. Added to this mixture at the aforementionedtemperature were 10.3 g of a 7% strength by weight aqueous solution ofsodium peroxodisulfate. After 3 minutes, feed streams 1 and 2 werestarted and were metered in at a uniform rate over 3 h.

Feed Stream 1 (An Aqueous Emulsion)

595.6 g deionized water 37.5 g a 32% strength by weight aqueous solutionof Disponil FES 77 (ethoxylated C12-C14 Na sulfate) 4.1 g a 58% strengthby weight aqueous solution of Lumiten I-SC (succinic ester) 825.0 gethylhexyl acrylate 267.0 g ethyl acrylate 24.0 g styrene 60.0 g methylacrylate 24.0 g acrylic acid

Feed Stream 2

92.6 g a 7% strength by weight aqueous solution of sodiumperoxodisulfate.

Feed Stream 3

12.2 g a 25% strength by weight ammonia solution and 24 g deionizedwater.

Feed Stream 4

12.0 g a 10% strength by weight aqueous solution of tert-butylhydroperoxide.

Feed Stream 5

16.0 g a 12% strength by weight aqueous solution of sodium acetonedisulfite.

Feed Stream 6

0.24 g Agitan LF 305.

Feed Stream 7

60.0 g deionized water.

After the end of feed streams 1 and 2, stirring was carried out for 30minutes and then pH was adjusted using feed stream 3.

Subsequently feed streams 4 and 5 were commenced and were metered in ata uniform rate over 60 minutes.

After the end of feed streams 4 and 5, feed stream 6 and feed stream 7were added.

The internal temperature of the reactor was lowered to 25° C. Theaqueous polymer dispersion obtained had a solids content of 53.6% byweight. The average particle size was 178 nm.

COMPARATIVE EXAMPLE 2 Hydroxyethyl Acrylate Metered in at a Uniform Rate

In a 4-liter polymerization reactor with anchor stirrer andheating/cooling apparatus a mixture of 223.9 g of deionized water and18.2 g of a 33% strength by weight aqueous polymer latex (prepared byfree-radically initiated emulsion polymerization of styrene) having aweight-average particle diameter Dw50 of 30 nm was heated at 85° C.under a nitrogen atmosphere. Added to this mixture at the aforementionedtemperature were 10.3 g of a 7% strength by weight aqueous solution ofsodium peroxodisulfate. After 3 minutes, feed streams 1 and 2 werestarted and were metered in at a uniform rate over 3 h.

Feed Stream 1 (An Aqueous Emulsion)

595.6 g deionized water 37.5 g a 32% strength by weight aqueous solutionof Disponil FES 77 (ethoxylated C12-C14 Na sulfate) 4.1 g a 58% strengthby weight aqueous solution of Lumiten I-SC (succinic ester) 825.0 gethylhexyl acrylate 243.0 g ethyl acrylate 24.0 g styrene 60.0 g methylacrylate 24.0 g acrylic acid 24.0 g hydroxyethyl acrylate

Feed Stream 2

92.6 g a 7% strength by weight aqueous solution of sodiumperoxodisulfate.

Feed Stream 3

12.2 g a 25% strength by weight ammonia solution and 24 g deionizedwater.

Feed Stream 4

12.0 g a 10% strength by weight aqueous solution of tert-butylhydroperoxide.

Feed Stream 5

16.0 g a 12% strength by weight aqueous solution of sodium acetonedisulfite.

Feed Stream 6

0.24 g Agitan LF 305.

Feed Stream 7

60.0 g deionized water.

After the end of feed streams 1 and 2, stirring was carried out for 30minutes and then pH was adjusted using feed stream 3. Subsequently feedstreams 4 and 5 were commenced and were metered in at a uniform rateover 60 minutes.

After the end of feed streams 4 and 5, feed stream 6 and feed stream 7were added. The internal temperature of the reactor was lowered to 25°C. The aqueous polymer dispersion obtained had a solids content of 53.4%by weight. The average particle size was 193 nm.

INVENTIVE EXAMPLE 1 Hydroxyethyl Acrylate in Stage

In a 4-liter polymerization reactor with anchor stirrer andheating/cooling apparatus a mixture of 223.9 g of deionized water and18.2 g of a 33% strength by weight aqueous polymer latex (prepared byfree-radically initiated emulsion polymerization of styrene) having aweight-average particle diameter Dw50 of 30 nm was heated at 85° C.under a nitrogen atmosphere. Added to this mixture at the aforementionedtemperature were 10.3 g of a 7% strength by weight aqueous solution ofsodium peroxodisulfate.

Feed Stream 1 (An Aqueous Emulsion)

595.6 g deionized water 37.5 g a 32% strength by weight aqueous solutionof Disponil FES 77 (ethoxylated C12-C14 Na sulfate) 4.1 g a 58% strengthby weight aqueous solution of Lumiten I-SC (succinic ester) 825.0 gethylhexyl acrylate 243.0 g ethyl acrylate 24.0 g styrene 60.0 g methylacrylate 24.0 g acrylic acid

Feed stream 1 is divided. Two portions are formed:

Feed Stream 1a

⅔ of feed stream 1

Feed Stream 1b

⅓ of feed stream 1 with additional 24 g 2-hydroxyethyl acrylate.

Feed Stream 2

92.6 g a 7% strength by weight aqueous solution of sodiumperoxodisulfate.

Feed Stream 3

12.2 g a 25% strength by weight ammonia solution and 24 g deionizedwater.

Feed Stream 4

12.0 g a 10% strength by weight aqueous solution of tert-butylhydroperoxide.

Feed Stream 5

16.0 g a 12% strength by weight aqueous solution of sodium acetonedisulfite.

Feed Stream 6

0.24 g Agitan LF 305.

Feed Stream 7

60.0 g deionized water.

After 3 minutes of addition of the amount of initiator, feed stream 1a(without hydroxyethyl acrylate) and feed stream 2 were commenced. Feedstream 1a (without HEA) was metered in over 2 h. Immediately after feedstream 1a (without HEA), feed stream 1b (with HEA) was commenced and wasmetered in over 1 h.

During the two feed streams 1a and 1b, feed stream 2 (initiator) wasmetered in over 3 hours.

After the end of feed streams 1a, 1b, and 2, stirring was carried outfor 30 minutes. Thereafter the pH was adjusted using feed stream 3.Subsequently feed streams 4 and 5 were commenced and were metered in ata uniform rate over 60 minutes. After the end of feed streams 4 and 5,feed stream 6 and feed stream 7 were added.

The internal temperature of the reactor was lowered to 25° C. Theaqueous polymer dispersion obtained had a solids content of 51.8% byweight. The average particle size was 207 nm.

COMPARATIVE EXAMPLE 3 Methacrylamide Metered in at a Uniform Rate, NoStage

In a 4-liter polymerization reactor with anchor stirrer andheating/cooling apparatus a mixture of 223.9 g of deionized water and18.2 g of a 33% strength by weight aqueous polymer latex (prepared byfree-radically initiated emulsion polymerization of styrene) having aweight-average particle diameter Dw50 of 30 nm was heated at 85° C.under a nitrogen atmosphere. Added to this mixture at the aforementionedtemperature were 10.3 g of a 7% strength by weight aqueous solution ofsodium peroxodisulfate. After 3 minutes, feed streams 1 and 2 werestarted and were metered in at a uniform rate over 3 h.

Feed Stream 1 (An Aqueous Emulsion)

563.0 g deionized water 37.5 g a 32% strength by weight aqueous solutionof Disponil FES 77 (ethoxylated C12-C14 Na sulfate) 4.1 g a 58% strengthby weight aqueous solution of Lumiten I-SC (succinic ester) 819.0 gethylhexyl acrylate 267.0 g ethyl acrylate 24.0 g styrene 60.0 g methylacrylate 24.0 g acrylic acid 40.0 g methacrylamide, 15% form

Feed Stream 2

92.6 g a 7% strength by weight aqueous solution of sodiumperoxodisulfate.

Feed Stream 3

12.2 g a 25% strength by weight ammonia solution and 24 g deionizedwater.

Feed Stream 4

12.0 g a 10% strength by weight aqueous solution of tert-butylhydroperoxide.

Feed Stream 5

16.0 g a 12% strength by weight aqueous solution of sodium acetonedisulfite.

Feed Stream 6

0.24 g Agitan LF 305.

Feed Stream 7

60.0 g deionized water.

After the end of feed streams 1 and 2, stirring was carried out for 30minutes and then pH was adjusted using feed stream 3.

Subsequently feed streams 4 and 5 were commenced and were metered in ata uniform rate over 60 minutes.

After the end of feed streams 4 and 5, feed stream 6 and feed stream 7were added. The internal temperature of the reactor was lowered to 25°C. The aqueous polymer dispersion obtained had a solids content of 53.2%by weight. The average particle size was 189 nm.

INVENTIVE EXAMPLE 2 Methacrylamide, Stage

In a 4-liter polymerization reactor with anchor stirrer andheating/cooling apparatus a mixture of 223.9 g of deionized water and18.2 g of a 33% strength by weight aqueous polymer latex (prepared byfree-radically initiated emulsion polymerization of styrene) having aweight-average particle diameter Dw50 of 30 nm was heated at 85° C.under a nitrogen atmosphere. Added to this mixture at the aforementionedtemperature were 10.3 g of a 7% strength by weight aqueous solution ofsodium peroxodisulfate. Feed stream 1 was divided ½ and 1/2. Feed 1 wasadded to ½ of feed stream 1.

Feed 2 was 40 g methacrylamide, 15% form.

After 3 minutes, feed stream 1 (without methacrylamide) and feed stream2 were commenced. Feed stream 1 without MAM was metered in over 1.5 hand feed stream 2 was metered in over 3 h. Immediately after feed stream1 without MAM, feed stream 1 with MAM was commenced and was metered inover 1.5 h.

Feed Stream 1 (An Aqueous Emulsion)

561.6 g deionized water 37.5 g a 32% strength by weight aqueous solutionof Disponil FES 77 (ethoxylated C12-C14 Na sulfate) 4.1 g a 58% strengthby weight aqueous solution of Lumiten I-SC (succinic ester) 819.0 gethylhexyl acrylate 267.0 g ethyl acrylate 24.0 g styrene 60.0 g methylacrylate 24.0 g acrylic acid

Feed Stream 2

92.6 g a 7% strength by weight aqueous solution of sodiumperoxodisulfate.

Feed Stream 3

12.2 g a 25% strength by weight ammonia solution and 24 g deionizedwater.

Feed Stream 4

12.0 g a 10% strength by weight aqueous solution of tert-butylhydroperoxide.

Feed Stream 5

16.0 g a 12% strength by weight aqueous solution of sodium acetonedisulfite.

Feed Stream 6

0.24 g Agitan LF 305.

Feed Stream 7

60.0 g deionized water.

After the end of feed streams 1 and 2, stirring was carried out for 30minutes. Thereafter the pH was adjusted using feed stream 3.

Subsequently the feed streams 4 and 5 were commenced and were metered inat a uniform rate over 60 minutes. After the end of feed streams 4 and5, feed streams 6 and 7 were added.

The internal temperature of the reactor was lowered to 25° C. Theaqueous polymer dispersion obtained had a solids content of 52.9% byweight. The average particle size was 185 nm.

TABLE 1 Composition of the polymers Comparative Comparative InventiveComparative Inventive example 1 example 2 example 1 example 3 example 2Ethylhexyl acrylate 68.75 68.75 68.75 68.25 68.25 Ethyl acrylate 22.2520.25 20.25 22.25 22.25 Methyl acrylate 5 5 5 5 5 Styrene 2 2 2 2 2Acrylic acid 2 2 2 2 2 Methacrylamide 0.5 0.5 (cont.) (staged)2-Hydroxyethyl 2 2 acrylate (cont.) (staged)

Agitan LF 305 is a mixture of liquid paraffin and nonionic emulsifiers.

Performance Testing

The pressure-sensitive adhesives were coated directly onto a polyesterfilm backing using a slotted doctor blade having a slot size of 60 pmand were dried at 90° C. for 3 minutes.

Procedure

Assessment: the visual appearance was assessed according to scores from0 to 4:

-   0: no clouding-   1: very slight clouding-   2: more severe clouding-   3: severe clouding-   4: very severe clouding

RESULTS

Immersion time in water in seconds (s), minutes (min), ComparativeComparative Inventive Comparative Inventive or hours (h) example 1example 2 example 1 example 3 example 2 10 s 0 0 0 0 0 30 s 0 0 0 0 0 60s 1 0 0 1 1  3 min 1 0 0 1 1  5 min 1 0 0 1 1 10 min 2 0 0 1 1 20 min 20 0 1 1 30 min 3 0 0 2 2 40 min 4 0 0 3 2 60 min 1 0 3 3  2 h 2 2 3 3  3h 3 2 3 3

1. A pressure-sensitive adhesive comprising an aqueous polymerdispersion prepared by emulsion polymerization, wherein the polymercomprises monomers containing a hydroxyl group or primary amino group,hereinafter referred to collectively as hydrophilic monomers, whereinthe polymer is produced by supplying more than 70% by weight of thehydrophilic monomers only when the polymerization mixture present in thepolymerization vessel and composed of monomers and polymer alreadyformed comprises more than 50% by weight of the total amount ofmonomers.
 2. The pressure-sensitive adhesive according to claim 1,wherein the polymer comprises at least 60% by weight of principalmonomers, selected from C1 to C20 alkyl (meth)acrylates, vinyl esters ofcarboxylic acids comprising up to 20 carbon atoms, vinylaromatics havingup to 20 carbon atoms, ethylenically unsaturated nitrites, vinylhalides, vinyl ethers of alcohols comprising 1 to 10 carbon atoms,aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two doublebonds, or mixtures of these monomers.
 3. The pressure-sensitive adhesiveaccording to claim 1, wherein the polymer is composed of at least 60% byweight of C1 to C20 alkyl (meth)acrylates.
 4. The pressure-sensitiveadhesive according to claim 1, wherein the hydrophilic monomers areselected from hydroxyalkyl (meth)acrylates, (meth)acrylamide or mixturesthereof.
 5. The pressure-sensitive adhesive according to claim 1,wherein the polymer comprises in total 0.1% to 15% by weight of thehydrophilic monomers.
 6. The pressure-sensitive adhesive according toclaim 1, wherein the polymer is obtainable by supplying more than 90% byweight of the hydrophilic monomers only when the polymerization mixturepresent in the polymerization vessel and composed of monomers andpolymer already formed already comprises more than 70% by weight of thetotal amount of monomers.
 7. The pressure-sensitive adhesive accordingto claim 1, wherein the polymerization mixture at the beginning ofaddition of the more than 70% by weight of the hydrophilic monomers iscomposed of at least 60% by weight of polymer already formed.
 8. Amethod for producing self-adhesive articles comprising employing thepressure-sensitive adhesive according to claim
 1. 9. An aqueous polymerdispersion prepared by emulsion polymerization, wherein the polymercomprises monomers containing a hydroxyl group or primary amino group,hereinafter referred to collectively as hydrophilic monomers, whereinthe polymer is produced by supplying more than 70% by weight of thehydrophilic monomers only when the polymerization mixture present in thepolymerization vessel and composed of monomers and polymer alreadyformed comprises more than 50% by weight of the total amount ofmonomers.
 10. The pressure-sensitive adhesive according to claim 6,wherein the polymerization mixture at the beginning of addition of themore than 90% by weight of the hydrophilic monomers is composed of atleast 60% by weight of polymer already formed.